Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the plate is exposed with an ultraviolet light from a lamp through a separate photomask film having predetermined imaging pattern that is placed between the light source and the plate, and the exposed plate is developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas.
Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength, allowing the elimination of the photomask film. Suitable lasers include, for example, infrared lasers (such as laser diode of about 830 nm and NdYAG laser of about 1064 nm), visible lasers (such as frequency-doubled NdYAG laser of about 532 nm and violet laser diode of about 405 nm), and ultraviolet laser (such as ultraviolet laser diode of about 370 nm).
Laser sensitive plates generally have higher sensitivity (than conventional plates) because of the limited laser power and the desire for fast imaging speed. Accordingly, photosensitive plates designed for laser imaging generally have limited room light stability. For example, before being developed to remove the non-hardened areas, frequency-doubled NdYAG laser sensitive plates usually require red room light for handling, violet laser sensitive plates usually require orange or yellow room light for handling, and infrared laser sensitive photopolymer plates usually require yellow room light for handling and have only limited white light stability (due to, for example, the use of certain initiator which has spectral sensitivity in the ultraviolet region).
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after imagewise exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing the reduction of labor and the elimination of hazardous waste. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.
An on-press developable plate is designed to be developed on a lithographic printing press in a pressroom, which is generally under regular office light (white room light). The imagewise exposed plate typically sees the white room light during the handling and on-press development. Therefore, the plate must be designed so that it is stable enough under regular office light within a certain time period (such as 30 minutes), and yet has sufficient photospeed suitable for imaging with available commercial imagers. However, a plate with faster photospeed generally has shorter handling window under white room light, while a plate with longer handling window under white room light generally has slower photospeed. Therefore, it is very difficult to design a laser sensitive on-press developable plate having both fast enough photospeed and wide enough handling window (in terms of time) under white room light.
It would be desirable if a laser sensitive on-press developable lithographic plate and/or method of using it can be designed so that the plate has sufficient photospeed during laser exposure and the laser exposed plate can be handled freely under white room light before and during on-press development.
The inventor has found that such a desire can be achieved by a method of removing the overcoat of a specific overcoated lithographic plate after imagewise exposing said plate and before mounting said plate on press for development with ink and/or fountain solution. Here said plate is capable of hardening upon exposure to a laser and has limited white room light stability before the removal of the overcoat, and has significantly improved white room light stability after the removal of the overcoat.